It is well known that the addition of soluble sulfides, such as sodium sulfide, polysulfides, liquid sulphur, etc. is effective in removing many heavy metal contaminants from waste waters, effluents and certain process solutions. Such sulfide additions convert heavy metals to extremely insoluble metal sulfide precipitates that can be eventually removed from the contaminated water by settling or filtration or both. Sulfide precipitation is often the only effective method for removing heavy metal contaminants that are bound by complexing or chelating compounds.
For maximum effectiveness of sulfide additions for heavy metals removal, both the amount of sulfide used and the solution pH must be controlled. If, for example, too little sulfide is added or an inaccurate pH is employed, removal of the metal contaminants will be incomplete. Since sulfide, itself, is a contaminant and also for obvious economic reasons, overdosing must be kept to a minimum. Thus, it is highly desirable to have a reliable process or system to accurately measure and control pH and the amount of sulfide being added to the process. It is also important that the process or system be unaffected adversely by normal fluctuations in composition, temperature, ionic strength, pH, etc. that are normally encountered in industrial waters and effluents. Although accurate and reliable instrumentation is currently available for control of pH, no instrumental system or process is known to exist for measurement and control of soluble sulfide concentrations.
In an attempt to overcome the problem associated with and inherent in the lack of reliable sulfide control, a process was developed for heavy metals removal based on the use of the very slightly soluble compound, ferrous sulfide. This process is set forth in U.S. Pat. No. 3,740,331 and in EPA publication 625/8-80-003. It has been used to remove certain heavy metals by converting them to insoluble sulfides while, at the same time, maintaining very low levels of excess soluble sulfides in the process. However, the major deficiency of this approach is that it produces very large quantities or iron and metal sulfide sludges that must be disposed of as hazardous waste. An EPA report 600/S2-81-081 on the Characterization of Boliden's Sulfide-Line Precipitation System has also documented the importance of providing a reliable sulfide control and the lack thereof in the following statements: "The full-scale results of a typical run are shown in FIG. 4. H.sub.g, Cu and Cd and Pb removals were complete by sulfide precipitation, whereas As and Zn separation was not adequate because of improper control of Na.sub.2 S dosage and pH." And again: "The inadequate separations (by sulfide precipitation only) of As and Zn obtained in the full scale tests were due to the wide fluctuations of pH and sulfide dosage . . . ."
Attempts have been made to use a soluble specific ion electrode immersed directly in solutions that require treatment with sulfide to control sulfide additions. EPA 600/2-80-139 on page 5, describes an attempt that has been made to use a sulfide specific ion electrode immersed directly in the solutions which require treatment with sulfides to control sulfide additions. It will be appreciated by those familiar with such electrodes that the signals generated are particularly affected by fluctuations in pH and ionic strength of the solution. Although the direct use of a sulfide ion electrode may work well in the laboratory under strictly or closely controlled and consistent conditions, it has been doomed to failure in industrial applications where conditions of pH and ionic strength constantly vary. ORP electrodes used in the control of sulfide are even more sensitive to changes in pH, ionic strength, as well as other variables.
There has thus been a real and definite need for an efficient and effective operating system and procedure to accomplish an accurate and reliable measurement and control of soluble sulfide concentrations that will be uneffected by changes in pH, ionic strength, temperature and other variables that are normally encountered in industrial waters and effluents.